When light hits a metal surface it can produce an electric potential. This phenomenon is called photoelectric effect. This concept is used to generate electricity directly from the energy of light. Solar Panels use this concept in full effect to charge the batteries of the calculators, as a source of power supply in traffic lights and also used as the source of light to light up our homes. Solar energy is one of the reliable sources of energy and it is the largest source of energy on planet earth.The use of solar energy helps in cutting down the power costs and to have a healthier environment.
Light of a sufficiently small wavelength can have enough energy to knock out some electrons from a metal surface. This is known as the photoelectric effect. In many ways, this is the opposite of atomic spectral emission where electrons from higher energy states lose energy and emit light. In the photoelectric effect, if a photon of light has sufficient energy, it can raise the electron to a higher energy level and even let it escape the atom’s grasp; in other words, emit an electron. When this happens, a potential difference develops between the atom and the electron, enabling us to use this effect to generate electricity.
Einstein’s equations for the photoelectric effect
According to the Einstein, the photoelectric effect is
Energy of photon = Energy needed to remove an electron + kinetic energy of the emitted electron
hν = W + E
h — Planck’s constant.
ν — Frequency of the incident photon.
W — Work function.
E — The maximum kinetic energy of ejected electrons: 1/2 mv².
Applications of the photoelectric effect:
- Used to generate electricity in Solar Panels. These panels contain metal combinations that allow electricity generation from a wide range of wavelengths.
- Motion and Position Sensors: In this case, a photoelectric material is placed in front of a UV or IR LED. When an object is placed in between the LED and sensor, light is cut off and the electronic circuit registers a change in potential difference
- Lighting sensors such as the ones used in smartphone enable automatic adjustment of screen brightness according to the lighting. This is because the amount of current generated via the photoelectric effect is dependent on the intensity of light hitting the sensor.
- Digital cameras can detect and record light because they have photoelectric sensors that respond to different colors of light.
- X-Ray Photoelectron Spectroscopy (XPS): This technique uses x-rays to irradiate a surface and measure the kinetic energies of the emitted electrons. Important aspects of the chemistry of a surface can be obtained such as elemental composition, chemical composition, the empirical formula of compounds and chemical state.
For this phenomenon to take place the photons should have the energy greater than or equal to the work function of the metal.
Where h is the plank’s constant, f is the frequency of the incident photon, and W is the work function of the metal used. The frequency which is equal to w/h is unique for a metal, it varies from metals to metal, because the work function of each metal is different from the other. When the photon is absorbed by a metal, it transfers a part of the energy to the breaking the electron free from its orbit and rest of the energy is converted into the kinetic energy of the electron itself, therefore the equation for this could be written as,
hν = W + E
Here h is the plank’s constant V is the frequency W is the energy required to remove an electron E is the maximum kinetic energy of the electron Now kinetic energy is given by 12 mv2 therefore we can find the velocity of the ejected electron by this equation. Look at the following problem, q) Electrons with kinetic energy 6.023 × 104J/mol are ejected from the surface of a metal, when exposed to a radiation of wavelength of 600nm, what is the minimum amount of energy required from the metal atoms.
When the radiation falls on a body some amount of energy is used to knock the electron out of the orbit, rest is used as the kinetic energy of the electron. Therefore,
Eradiation = W + KE
We know kinetic energy per mol lets measure the kinetic energy per electron, There are 6.023 × 1023 electrons in 1 mole of matter therefore, The kinetic energy of 1 electron would be, 6.023 × 1046.023 × 1023 = 1 × 10−19J
Let’s measure the energy of each photon,
Eradiation = hcλ 6.626 × 10−34 Js × (3 × 108)ms−1600 × 10−9m = 3.313 × 10−19J
Now we can equate the two values in the equation and find the unknown work function,
W = Eradiation − KE W = 3.313 × 10−19 − 1 × 10−19 = 2.313 × 10−19 J
Therefore the work function of the metal is found to be 2.313 × 10−19 Joules, the same question can be asked by giving us the work function and asking us to find the velocity at which the electron is ejected, this could be solved with the same equation and the formula for Kinetic energy.
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